Field effect transistors are known in which source and drain electrodes are bridged by a semiconductor the conductivity of which is controlled by a gate electrode insulated from the semiconductor and the source and drain electrodes to which a potential is applied for this purpose.
In order to meet performance specifications it is necessary for such devices to be acceptably consistent in their performance. This is particularly important in display devices containing many transistors in which inconsistent performance can show as defects in the display.
US 2010/0155710 discloses the process of forming well oriented crystals by isotropic growth and the large crystals of the semiconductor in the channel between the source and drain electrodes appear in the photographs to bridge the channel. This is achieved by depositing a drop of water on its drain electrode and depositing an immiscible solution of a semiconductor on to the drop of water which solution flows over the water droplet into a surrounding channel between drain electrode and the source electrode. The channel seems necessarily to surround the drain electrode but this is difficult to reconcile with some of the drawings and with the photograph of FIG. 9 in which the source and drain electrodes are said to be square shaped.
The procedure seems to have been carried out on a small scale using pipettes or the like and the channels were 200 μm long in the direction of current flow. There are difficulties in scale up to industrial production and in dealing with the short channel lengths of as little as 4 μm now being adopted.
Problems mentioned as requiring attention in the specification include liquid amounts, liquid viscosity, liquid evaporation rates, drop height, drop angle, drop atmosphere, drop splash etc. The problem of delivering droplets onto very tiny targets and then delivering second droplets onto the top of the first droplet are believed to be beyond the practicality of presently known equipment. The problem of preventing variations in the size of the droplets impacting the target and other difficulties are likely to introduce non-uniformity in transistors. The process described appears to be of great difficulty and there is a need for a simpler process of producing transistors of closely reproducible performance on a large scale.
A further problem with the products of this process is that a substantial part of the drain electrode would not be covered by the semiconductor although the degree to which this would be the case would presumably depend on the extent to which the impact of the semiconductor solution splashed or spread the water over the surface. If the water spread into the channel as a result the semiconductor would presumably not fill the channel and its evaporation might damage the connection.
This invention enables the large-scale production of transistors of satisfactorily uniformity of properties by a simpler procedure using semiconductors in which the crystallites do not have to be well oriented.
In WO 03/058729 semiconductor devices are described in which the channel separating the source and drain electrodes is extended by adopting what is described as a non-linear formation. In most of the drawings the non-linear channels are composed of connected linear sections and no curvature is present. Such an arrangement can achieve close packing in which the width of the channel can be maximised. In accordance with the convention used in this art we define channel length as being the distance between source and drain electrodes and channel width to be the geometrical extent of the said channel.
There is no teaching relating to the crystalline nature of the semiconductors or uniformity of performance of transistors, in this reference but in order to emphasise the generality the invention in FIG. 6 a serpentine configurations is shown. This appears to be a non-optimum use of the available space.